Many metallic substrates, such as those including aluminum alloys, may be anodized to increase corrosion resistance and wear resistance of the substrate. Anodizing is an electrolytic passivation process used to increase the thickness and density of the natural oxide layer on the surface of metal parts. Anodic films can also be used for a number of cosmetic effects, either via thick porous coatings that can absorb dyes or via thin transparent coatings that add interference effects to reflected light. Anodic films are generally much stronger and more adherent than most paints and platings, making them less likely to crack and peel. Anodic films are most commonly applied to protect aluminum alloys, although processes also exist for titanium, zinc, magnesium, and niobium.
With respect to aluminum alloys, during anodizing an aluminum oxide coating is grown from and into the surface of the aluminum alloy in about equal amounts, so, for example, a 2 μm thick coating will increase part dimensions by 1 μm per surface. Anodized aluminum alloy surfaces can also be dyed. In most consumer goods the dye is contained in the pores of the aluminum oxide layer. Anodized aluminum surfaces have low to moderate wear resistance, although this can be improved with thickness and sealing. If wear and scratches are minor then the remaining oxide will continue to provide corrosion protection even if the dyed layer is removed.
While conventional anodizing processes may yield anodized substrates having good abrasion resistance and ability to color the surface with dyes, such substrates are not without their drawbacks. For instances, many anodized substrates are unable to provide durability and chemical stability in a corrosive environment, and also are generally unable to provide hydration stability in humid and outdoor environments. Protective compounds may be applied to the anodized surfaces, but it is difficult to maintain adhesion and chemical compatibility of these protective compounds with anodized surfaces while maintaining suitable abrasion resistance and coloring ability. In turn, the overall performance of the corresponding finished products may be inadequate for certain applications.